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. 2025 Aug 26;10(35):40668–40674. doi: 10.1021/acsomega.5c07919

Selectively Adsorbed CO and O2 on Transition-Metal-Incorporated Porphyrin

Janghwan Cha †,‡, Hoonkyung Lee §, Suklyun Hong †,*
PMCID: PMC12423866  PMID: 40949267

Abstract

We have investigated the adsorption of CO and O2 on transition-metal (TM)-incorporated porphyrin. Our calculations show that one or more CO molecules can be adsorbed on the Sc-, Ti-, V-, Cr-, or Mn-incorporated porphyrin, while only one CO molecule is adsorbed on the Fe- or Co-incorporated porphyrin. In the case of O2, only one O2 molecule prefers to adsorb on the TM-incorporated porphyrin, regardless of TM. Moreover, O2 is strongly bound to the Sc-, Ti-, V-, Cr-, or Mn-incorporated porphyrin compared with that of CO, while it is weakly bound to the Fe- or Co-incorporated porphyrin compared with that of CO. Such different binding behaviors can be explained by variations of the highest occupied molecular orbital (HOMO) level in the d-orbitals of TM-incorporated porphyrin. We find that the HOMO levels are shifted downward in the cases of O2 adsorption on the Sc-, Ti-, and V-incorporated porphyrin and CO adsorption on the Fe- and Co-incorporated porphyrin, indicating the stronger binding energies between them. These studies can be useful to understand the capture of CO and O2 by the TM-incorporated porphyrin.

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I. Introduction

Hemoglobin, which constitutes the blood of vertebrates breathing with lungs or gills, consists of α and β proteins and four heme structures. , Heme structure is the pivotal structure that causes hemoglobin to carry oxygen (O2). Among the components constituting the heme, Fe stores and carries O2. The heme structure also exists in myoglobin, which plays a role in storing O2 in the muscles of vertebrates. , The red color of the blood and muscles of vertebrate animals is due to Fe of the heme structure. Hemoglobin, which carries O2 throughout the body from the lungs or gills, can also carry carbon dioxide (CO2), another gas molecule other than O2: It carries some of the body’s respiratory CO2. The binding between CO2 and hemoglobin is weaker than that between O2 and hemoglobin. However, myoglobin in muscle has a stronger binding energy to O2 than hemoglobin in the blood, so O2 molecules move from the blood to the muscle. Conversely, myoglobin has a weaker binding energy to CO2 than hemoglobin, so CO2 is transferred from the muscle to the blood.

When a carbon compound is incompletely burned, carbon monoxide (CO) can be generated, along with CO2. The concentration of CO is high in the factory area or in areas where vehicles or aircraft are used heavily. CO binds to the hemoglobin of red blood cells instead of to O2 because the binding energy of CO is higher than that of O2 when CO reaches the blood through respiration. The resultant product is called carboxyhemoglobin. , Carboxyhemoglobin is life-threatening because it lowers the overall O2-carrying capacity of the blood. Since CO is tasteless, odorless, and colorless, it is difficult to detect poisoning early on. It is generally known that CO has a 200 times more affinity to hemoglobin than O2. Therefore, the reaction mechanism between hemoglobin heme and molecules, such as CO, O2, CO2, and NO, has been studied. Especially, many computational studies with Fe-incorporated porphyrin have been carried out − because it has a similar structure to the heme existing in hemoglobin and myoglobin. In addition, the adsorption of gas molecules on porphyrin containing other transition metals (TMs) instead of Fe has been studied. − Other studies of TMs adsorbed on carbon-based materials such as graphene, graphene nanoribbons, and coronene similar to porphyrins have also been reported.

Porphyrin is a group of heterocyclic macrocycle organic compounds consisting of a modified subunit of four pyrrole (C4H4NH) linked at the α carbon atom via a methine bridge (CH−). Four nitrogen atoms are located at the center of porphyrin, two nitrogen atoms are H-passivated, and the remaining two N atoms do not have H. When H is dissociated from the H-passivated nitrogen atom, a cation metal can be adsorbed to the N atoms at the center of the porphyrin.

In this study, we investigated the binding behaviors of CO and O2 onto porphyrin with 3d TM from the viewpoint of a deeper understanding of the molecular reactions with the heme structure. Through the calculations, we identify which TM-incorporated porphyrins (TM-porphyrins) are more strongly bonded to either CO or O2. The trend of binding energies is explained by variations of the highest occupied molecular orbital (HOMO) level in the d-orbitals of TM-porphyrin.

2. Computational Method

For computations, we used the Vienna Ab initio Simulation Package (VASP) and the Gaussian 09 package (G09). To optimize atomic structures of CO and O2 adsorbed on TM-porphyrins, density functional theory (DFT) calculations are performed within generalized gradient approximation (GGA) for exchange-correlation (xc) functionals, , as implemented in VASP. , The kinetic energy cutoff is set to 400 eV, and electron–ion interactions are represented by the projector augmented wave (PAW) potentials. , Atomic coordinates are fully optimized until the Hellmann–Feynman forces are less than 0.01 eV/Å. The convergence criterion for the total energy is 10–5 eV. To ensure a distance of more than 10 Å between each TM-porphyrin structure, we used a 25 Å × 25 Å × 20 Å cubic unit cell. Next, we calculate molecular orbitals (MOs) of CO and O2 adsorbed on TM-porphyrin within the Perdew–Burke–Ernzerhof (PBE) version of the gradient-corrected exchange-correlation functional, as implemented in the G09 program. Valence double-ζ polarization with diffuse functions basis set (6-31+G*) is used. From the optimized geometries (by VASP) for the adsorbed CO and O2 on TM-porphyrin, we recalculate the optimized structures using G09. The optimized geometries are quite similar, as shown in both VASP and G09 calculations: The difference in the bond length of adsorbed molecules is less than 1%, while the difference in the distance from adsorbed molecules onto TM is also less than 1% except for the Fe case where the difference is about 3%.

3. Results and Discussion

First, we start with the TM-porphyrin structures, where 3d TMs such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn are incorporated onto the porphyrin molecules. To calculate the binding energies of CO molecules adsorbed on TM-porphyrin, three structures are considered as shown in Figure a–c, where the bonding configurations of CO-adsorbed TM-porphyrin are that the carbon or oxygen atom of the CO molecule is bonded to TM (see Figure a,b), while both carbon and oxygen atoms of the CO molecule are bonded to TM (see Figure c). Similarly to the CO-adsorbed case, when O2 molecules are adsorbed onto TM-porphyrin, three structures are also considered as in Figure d–f: two O atoms of O2 molecule are bonded to TM (see Figure d), while O2 molecule is located vertically or obliquely with one O atom bonded to TM (see Figure e,f).

1.

1

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Possible geometric structures of CO or O2 molecules adsorbed on TM-porphyrin. The upper and the lower structures show the top and side views of CO- or O2-adsorbed TM-porphyrin, respectively. The purple, gray, red, blue, and sky-blue balls represent TM, carbon, oxygen, nitrogen, and hydrogen atoms, respectively. The bonding configurations of CO-adsorbed TM-porphyrin: (a) C atom bonded to TM, (b) O atom bonded to TM, and (c) C and O atoms bonded to TM; those of O2-adsorbed TM-porphyrin: (d) two O atoms bonded to TM, (e) one O atom vertically bonded to TM, and (f) one O atom obliquely bonded to TM.

The binding energy E b of adsorbate (CO or O2) on the TM-porphyrin structure is obtained by the following equation:

Eb=−1n[Etotal−(ETM‐porphyrin+n×Eadsorbate)]

where E total, E TM‑porphyrin, and E adsorbate indicate the total energies of adsorbate/TM-porphyrin, TM-porphyrin, and adsorbate, respectively, and n represents the number of adsorbates. Note that the molecule is considered to be easily adsorbed when its binding energy is a positive value; that is, higher positive values indicate stronger binding.

The binding energies of n COs (n = 1, 2, 3, 4) adsorbed on TM-porphyrin are listed in Table along with those of O2: the binding energy corresponding to the most stable configuration among many possible ones is shown for each adsorbate such as CO and O2.

1. Binding Energies (per Molecule) of CO and O2 Adsorbed on TM-Porphyrins.

  CO on TM-porphyrin (eV/CO)
 O2 on TM-porphyrin (eV/O2)
atom 1 CO 2 CO 3 CO 4 CO 1 O2
Sc 0.96 0.67 0.47 0.32 2.96
Ti 1.52 0.94 0.64 0.36 4.22
V 1.45 0.90 0.49 0.25 2.91
Cr 0.42 0.33 0.03 0.03 1.16
Mn 1.06 0.33 0.03 0.03 1.09
Fe 1.50 0.08 –0.13 –0.12 0.78
Co 0.83 –0.29 –0.18 0.02 0.68
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When CO is adsorbed on TM-porphyrin (TM = Sc, Ti, V, Cr, Mn, Fe, and Co), the binding energies of CO in the configuration shown in Figure a are larger than those in Figure b,c. More specifically, the binding between TM-porphyrin and the C atom of CO is stronger than that between TM-porphyrin and the O atom of CO.

The binding energies of one CO molecule with TM = Sc, Ti, V, Cr, Mn, Fe, and Co of TM-porphyrin are 0.96, 1.52, 1.45, 0.42, 1.06, 1.50, and 0.83 eV per molecule, respectively. Note that the CO or the O2 molecule does not interact with TM-porphyrin with TM = Ni, Cu, and Zn.

For the O2-adsorbed case, it is found that the binding of one O2 molecule (i.e., two oxygen atoms) with TM = Sc, Ti, V, Cr, and Mn of TM-porphyrin is strongest for the structure shown in Figure d. The binding energies of O2 with Sc, Ti, V, Cr, and Mn are 2.96, 4.22, 2.91, 1.16, and 1.09 eV per molecule, respectively. In contrast, the strongest binding configuration of O2 with TM = Fe and Co is that of Figure f, where the molecule of the O2 is obliquely bonded to TM-porphyrin. The binding energies of the compound of O2 with Fe- and Co-porphyrin are 0.78 and 0.68 eV per molecule, respectively.

As shown in Table and Figure , the binding energies of the CO and O2 molecules adsorbed on TM-porphyrin are compared with each other. When TM is Sc, Ti, V Cr, and Mn, the binding energy of the O2 molecule adsorbed on TM-porphyrin is higher than that of the CO molecule. For example, the O2 binding energy adsorbed on Sc-porphyrin is 2.96 eV, which is higher than the CO binding energy (0.96 eV) on Sc-porphyrin. In contrast, when TM is Fe and Co, the CO binding is stronger than the O2 binding. Note that the O2 binding energy (0.78 eV) on Fe-porphyrin is lower than that of the 1 CO molecule (1.50 eV), which explains CO poisoning.

2.

2

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Calculated binding energy (per molecule) of CO and O2 molecules adsorbed on TM-porphyrin. The red, blue, green, orange, and yellow bars represent the binding energies for one O2, one CO, two CO, three CO, and four CO molecules adsorbed on TM-porphyrin, respectively. Multiple CO molecules can be adsorbed onto TM, while only one O2 can be.

We increased the number of CO or O2 molecules adsorbed on TM-porphyrin. It is found that only one O2 molecule can be adsorbed on TM-porphyrin regardless of TM; that is, more than one O2 molecule is difficult to bind to TM-porphyrin. In contrast, the number of adsorbed CO molecules varies depending on TM. For the case of TM = Sc, Ti, and V, up to four CO molecules can be bound to TM-porphyrin, while for TM = Cr and Mn, up to two CO molecules are adsorbed on TM-porphyrin.

In the case of 1 CO adsorption on TM-porphyrin, note that for TM = Sc, Ti, V, and Cr, the CO binding energy is smaller than the O2 binding energy and for TM = Mn, the CO binding energy is very close to that of O2 molecule, while for TM = Fe and Co, the CO binding energy is larger than that of O2. On the other hand, for the adsorption of more than one CO molecule on TM-porphyrin, the binding energy per CO molecule is much smaller than that of O2, as expected.

Next, we investigated the MO energy diagram for TM = Sc, Cr, and Fe to study the mechanism of how CO or O2 is adsorbed on TM-porphyrin. Three TMs, Sc, Cr, and Fe, are selected as the representatives of the category with the number of CO molecules being 4, 2, and 1, respectively, on TM-porphyrin.

Figure a–c shows the MO energy diagrams for (a) Sc-porphyrin, (b) CO-adsorbed Sc-porphyrin, and (c) O2-adsorbed Sc-porphyrin, respectively. The spin magnetic moment (i.e., magnetization) of all of the structures is 1 μB. The left-hand side of each MO energy diagram is for the majority spin, while its right-hand side is for the minority spin. The wave function characters of MOs in Figure d–f correspond to the energy states indicated by the arrows in Figure a–c, respectively.

3.

3

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MO energy diagram of (a) Sc-porphyrin, (b) CO-adsorbed Sc-porphyrin, and (c) O2-adsorbed Sc-porphyrin. Wave function characters of HOMO levels of (d) Sc-porphyrin and (e) CO-adsorbed Sc-porphyrin, and (f) HOMO – 2 level of O2-adsorbed Sc-porphyrin. The solid and dashed lines in (a)–(c) represent occupied and unoccupied states of the pristine and CO- and O2-adsorbed systems, respectively. The arrows in (a)–(c) indicate MOs of which wave function characters are given in (d)–(f), respectively.

In general, the energy levels of d-orbital TM are split by the crystal field theory (CFT). , The d-orbital is decomposed into t2g states (d xy , d yz , and d zx ) and eg states (d x 2–y 2 and d z 2 ). The structure of TM-porphyrin is a square planar structure; therefore, d yz and d zx orbitals (t2g states) behave as lower energy states than other d-orbitals. Therefore, HOMO of Sc-porphyrin is the d yz orbital of Sc, as shown in Figure d.

Figure e,f shows the hybridization of MOs when CO and O2 are adsorbed on Sc-porphyrin, respectively. Figure e shows the hybridization between the d yz atomic orbital of the Sc atom and ppπ* MO of the CO molecule in the HOMO level of CO-adsorbed Sc-porphyrin. However, in Figure f, we find the hybridization between the d yz orbital of Sc and ppπ* of O2 is located below the HOMO level (at the HOMO – 2 level) of O2-adsorbed Sc-porphyrin, rather than at the HOMO level. In other words, among MOs that consist of both the O2 and Sc orbitals, the HOMO – 2 level is the one closest in energy to HOMO. HOMO of the O2-adsorbed Sc-porphyrin is composed solely of porphyrin orbitals, without contributions from the Sc atom or the O2 molecules.

Comparison of Figure a,b shows that their occupied levels are similar. Especially, the energy gaps between the HOMO and the lowest unoccupied molecular orbital (LUMO) levels are 0.197 and 0.171 eV, respectively, which are comparable. This suggests that additional CO adsorption is possible through occupation of the vacant t2g state level, since the LUMO level of CO-adsorbed Sc-porphyrin lies close to its HOMO level. In contrast, the orbital hybridization between O2 and Sc occurs at the HOMO – 2 level rather than at HOMO (see Figure c), and HOMO of O2-adsorbed Sc-porphyrin is composed solely of MOs of the Sc-porphyrin itself. It is therefore expected that O2 binds more strongly to Sc-porphyrin than CO, as evidenced by the larger HOMO–LUMO gap of 0.777 eV.

Figure a–c shows the MO energy diagrams for (a) Cr-porphyrin, (b) CO-adsorbed Cr-porphyrin, and (c) O2-adsorbed Cr-porphyrin, respectively. The magnetization in Figure a is 4 μB, while that in Figure b,c is 2 μB. The energy gaps between the HOMO and the LUMO levels are 1.367 eV for Cr-porphyrin, 0.196 eV for CO-adsorbed Cr-porphyrin, and 0.715 eV for O2-adsorbed Cr-porphyrin. The wave function characters of MOs in Figure d–f correspond to the energy states indicated by the arrows in Figure a–c, respectively. Figure d shows the d yz atomic orbital of Cr in HOMO of the Cr-porphyrin structure. Note that the wave function characters shown in Figure d of Sc-porphyrin and Figure d of Cr-porphyrin are different from each other in terms of orbital hybridization. While HOMO of Sc-porphyrin comes from the bonding between the p orbital of the N atom of porphyrin and the d yz orbital of the Sc atom, HOMO of Cr-porphyrin comes from the antibonding between the p orbital of the N atom and the d yz orbital of the Cr atom.

4.

4

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MO energy diagram of (a) Cr-porphyrin, (b) CO-adsorbed Cr-porphyrin, and (c) O2-adsorbed Cr-porphyrin. Wave function characters of HOMO levels of (d) Cr-porphyrin, (e) CO-adsorbed Cr-porphyrin, and (f) O2-adsorbed Cr-porphyrin. For the explanation related to the solid and dashed lines and arrows, refer to Figure .

Figure e represents the wave function character of the HOMO level of CO-adsorbed Cr-porphyrin, where the d yz orbital of Cr in HOMO of Cr-porphyrin and the ppπ* orbital of CO are hybridized. Note that the hybridized MO between Cr-porphyrin and CO in Figure e is similar to that between Sc-porphyrin and CO in Figure e. In addition, CO is expected to be further adsorbed, since the LUMO level of CO-adsorbed Cr-porphyrin is close to its HOMO level.

Figure e,f shows the hybridization of MOs when CO and O2 are adsorbed on Cr-porphyrin, respectively. Especially, we find that the HOMO level between d x 2–y 2 of Cr and ppπ* of O2 has an antibonding character, as shown in Figure f. It is presumed that the binding of O2 on Cr-porphyrin is weaker than that on Sc-porphyrin because of the antibonding character in the HOMO level.

Figure a–c shows the MO energy diagrams for (a) Fe-porphyrin, (b) CO-adsorbed Fe-porphyrin, and (c) O2-adsorbed Fe-porphyrin. The magnetization in Figure a,c is 2 μB and that in (b) is 0. The energy gaps between the HOMO and LUMO levels are 0.320 eV for Fe-porphyrin, 1.686 eV for CO-adsorbed Fe-porphyrin, and 0.393 eV for the O2-adsorbed Fe-porphyrin. The wave function characters of MOs in Figure d–f correspond to the energy states indicated by the arrows in Figure a–c. Figure d shows the d yz orbital of Fe in HOMO of the Fe-porphyrin structure.

5.

5

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MO energy diagram of (a) Fe-porphyrin, (b) CO-adsorbed Fe-porphyrin, and (c) O2-adsorbed Fe-porphyrin. Wave function characters of (d) HOMO of Fe-porphyrin, (e) HOMO – 1 of CO-adsorbed Fe-porphyrin, and (f) HOMO of O2-adsorbed Fe-porphyrin. For the explanation related to the solid and dashed lines and arrows, refer to Figure .

Figure e represents the wave function character of the HOMO – 1 level of the CO-adsorbed Fe-porphyrin structure, which is similar to that of the HOMO level of the CO-adsorbed Cr-porphyrin shown in Figure e. Only the d yz orbital of Fe appears at the HOMO level of CO-adsorbed Fe-porphyrin. The reason why Fe forms a stronger bond with CO than Sc or Cr is that the hybridized MO between CO and Fe-porphyrin is located at the HOMO – 1 level in Figure e. Figure f shows the HOMO level of O2-adsorbed Fe-porphyrin. The binding is weakened because the d yz orbital of Fe and ppπ* of O2 form antibonding, similarly to the case of Cr.

From the calculated HOMO–LUMO gaps, the relative resistance changes upon CO adsorption can be estimated using R ∝ e E g/2k B T at 300 K, where E g denotes the HOMO–LUMO gap. For Sc-porphyrin, the gap decreases slightly from 0.197 to 0.171 eV, corresponding to a resistance drop to about 60% of its initial value, indicating a moderate conductivity enhancement. For Cr-porphyrin, the gap decreases drastically (1.367 → 0.196 eV), implying an extremely large conductivity increase. In contrast, Fe-porphyrin shows a large gap increase (0.320 → 1.686 eV), resulting in a resistance rise by many orders of magnitude, indicating strong suppression of conduction. These examples demonstrate that CO adsorption can lead to either enhanced or suppressed electrical conduction, depending on the transition metal. For reference, detailed resistance ratios for CO and O2 adsorption are provided in Table S1 of the Supporting Information.

Finally, to investigate the mechanism of CO and O2 adsorption on TM-porphyrin, a Mulliken charge analysis is performed. To examine the electron transfer from TM to porphyrin upon gas adsorption, the number of electrons in TM before and after CO (or O2) adsorption is compared with that in the TM + CO (or TM + O2) complex after adsorption. In the TM-porphyrin structure, it is found that 1.59e (Sc), 1.10e (Cr), and 0.66e (Fe) are transferred from the TM atom to the porphyrin. In the CO-adsorbed TM-porphyrin structure, 1.15e (Sc), −0.37e (Cr), and −1.55e (Fe) are transferred from TM + CO to the porphyrin, respectively, where a negative value indicates electron transfer from the porphyrin to TM + CO. Compared with the TM-porphyrin without CO, the amount of electron transfer to the porphyrin is reduced by 0.44e (Sc), 1.47e (Cr), and 2.21e (Fe). This reduction generally correlates with stronger CO binding to TM-porphyrin, although the relationship is not strictly proportional. In the O2-adsorbed TM-porphyrin structure, 0.78e (Sc), 2.13e (Cr), and 2.03e (Fe) are transferred from TM + O2 to the porphyrin. Compared with the TM-porphyrin without O2, the electron transfer to the porphyrin changes by 0.81e (Sc), −1.03e (Cr), and −1.37e (Fe). These values indicate that, for O2 adsorption on TM-porphyrins where TM is Cr or Fe, the TM + O2 bond is weakened due to electron transfer from TM + O2 to the porphyrin.

Through this study, we confirm that the selective capture of CO and O2 using TM-porphyrin is possible: TM-porphyrin structures with TM = Sc, Ti, and V are advantageous to O2 capture, while those with TM = Fe and Co are advantageous to CO capture. The observed HOMO–LUMO gap changes and corresponding resistance variations reinforce this selectivity and provide useful guidelines for identifying candidates for CO and O2 capture and detection applications.

4. Conclusions

We studied the atomic structure and binding mechanism of CO and O2 adsorbed on porphyrin with 3d TMs. In the case of TM = Ni, Cu, or Zn, the CO or O2 molecule is not likely to be adsorbed on TM-porphyrin because the d-orbitals fill the occupied level. For 3d TMs, such as Sc, Ti, V, Cr, Mn, Fe, and Co, the number of CO molecules adsorbed on TM-porphyrin is varied depending on TM: four COs (TM = Sc, Ti, V), two (TM = Cr, Mn), or one (TM = Fe, Co). In contrast, only one O2 molecule can be adsorbed on TM-porphyrin regardless of TM. For the case of adsorption of more than one CO molecule on TM-porphyrin with TM = Sc, Ti, V, Cr, and Mn, the binding energy per CO molecule is much lower than that of O2. Interestingly, when only one CO molecule is adsorbed on TM-porphyrin with TM = Fe and Co, the binding energy of the CO molecule is higher than that of the O2 molecule. Such different binding behaviors can be explained by variations in the location of the HOMO level in the d-orbitals of TM-porphyrin.

These results would advance our understanding of molecular reactions with biological units, such as the heme structure, and suggest that HOMO–LUMO gap changes and associated resistance variations can be used as simple indicators of selective gas capture.

Supplementary Material

ao5c07919_si_001.pdf (94.7KB, pdf)

Acknowledgments

This work was supported by the NRF (RS-2024-00355708 and RS-2024-00436306) and the IITP (RS-2024-00437191) funded by the Ministry of Science and ICT (MSIT), Korea. Some calculations and analyses were performed using the TGM-DFT platform of Antlinesoft.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c07919.

  • Calculation method for relative resistance changes of TM-porphyrins upon CO and O2 adsorption, Table S1 summarizing HOMO–LUMO gaps and resistance change ratios, and discussion of adsorption effects (PDF)

J.C.: Data curation, formal analysis, methodology, visualization, writingoriginal draft. H.L.: Conceptualization, formal analysis. S.H.: Conceptualization, formal analysis, funding acquisition, project administration, resources, supervision, writingreview and editing. All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.

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